1. Field of the Invention
The present invention relates in general to measurement systems using gas or particle detectors, such as those associated with mass spectrometry. More particularly, the invention relates to inductively coupled plasma mass spectrometry.
2. Background
Measurement systems utilizing gas or particle detectors, such as mass spectrometers, are widely known and widely used. For example, the semiconductor, environmental, geological, chemical, nuclear, clinical, and research industries all use measurement systems for a variety of composition detection. In particular, the semiconductor industry uses measurement systems for impurity analysis of many of the solutions used in the wafer fabrication process.
In a measurement system based on inductively coupled plasma mass spectrometry, the measurement system often employs a nebulizer, a spray chamber, an inductively coupled plasma torch, and a mass spectrometer. The nebulizer connects to the spray chamber. The spray chamber, in turn, is connected to the inductively coupled plasma torch. In one approach, connection tubing transfers the output of the spray chamber to the inductively coupled plasma torch. The output of the inductively coupled plasma torch, in turn, is connected to the mass spectrometer.
In general, conventional measurement systems direct a sample into the nebulizer which in turn, transforms the sample into a vapor or aerosol. The spray chamber then filters out some of the larger sample droplets in the aerosol. The remaining smaller sample droplets in the aerosol are transported by the connection tubing to the plasma torch. The plasma torch uses high-energy plasma to convert the sample into ionized atoms. The ionized atoms pass to the mass spectrometer and the mass spectrometer identifies the characteristics of the sample.
The sensitivity of conventional measurements systems is at least in part dependent on the quality and quantity of the sample which eventually reaches the mass spectrometer. To that end, designers have created measurement systems that employ carrier gases to help transport the sample. For example, carrier gases have been added to the nebulizer to try to provide uniformity in droplet size. Moreover, various carrier gases have been added to the plasma torch.
Unfortunately, many measurement systems still have drawbacks that can affect their accuracy and sensitivity. For example, after the spray chamber removes the larger sample droplets from the aerosol, the smaller droplets tend to be unstable. Instability can cause the smaller droplets to conglomerate back into larger droplets during passage through the connection tubing. In such cases, the larger reformed droplets can be trapped in the connection tubing and not only fail to reach the plasma torch, but also block properly sized droplets from passage. Thus, when large droplets form in the connection tubing, the mass spectrometer may receive fewer sample particles for analysis. Moreover, if some of the larger reformed droplets reach the plasma torch, they can distort the measurements performed by the mass spectrometer.
All of these drawbacks can cause measurement systems to provide errant impurity conclusions about the sample. In the semiconductor industry, where the samples are often solutions used in fabrication processes, such errant conclusions can lower semiconductor process yields and increase the overall cost of semiconductor manufacturing.